Financial support from the National Academy of Science of Ukraine (Grant No. 4/10-H “Quantum phenomena in nanosystems and nanomaterials at low temperatures”) is gratefully acknowledged. The research of E.N.B. and U.L. was supported by the Office of Basic Energy Sciences of the US Department of Energy under Grant No. FG05-86ER45234. I.V.K. acknowledges the hospitality of the School of Physics at the Georgia Institute of Technology (Atlanta).